This invention relates to acoustic telemetry in a borehole and more particularly to acoustically transmitting data over a pipe suspended in a borehole using precise frequencies of transmission. The problem of borehole telemetry has prevailed in the petroleum industry for a number of years. This problem has become increasingly crucial with the advent of deeper drilling, increased activity offshore, and rapidly escalating costs of drilling, all of which have brought about the requirements for drilling safer and less expensively. The acquisition of real time data from the bottom of a wellbore and in particular data associated with the parametric conditions of a drill bit offers the greatest potential for utilizing such a system. Recent increased attention to our energy needs has brought about the need for deeper drilling as well as increased activity in higher cost offshore and hostile environments. Basic drilling costs have escalated 150% in the last decade. These energy needs and escalating costs have focused attention on all potential methods to drill safer and cheaper. Of the possible methods, real time data from the drill bit offers the greatest potential to improved drilling efficiency and effectiveness in both exploration and production wells. The search for a reliable and economical method of obtaining information from the vicinity of the bit while drilling has been a goal ever since the advent of rotary drilling. Except in very special circumstances, however, previous attempts to develop real time measurewhile-drilling (MWD) systems have not met with success. Current technology is limited to surface evidence of drilling effectiveness. For example, measurements are presently made during drilling which include rotation rate, penetration rate, torque, etc. Also measured at the surface are the properties of the drilling fluid; i.e., weight, viscosity, etc. Systems are available to take the surface measurements and convert the information from analog to digital form, then process and display the parameters along with information inferred from them. Where experience is sufficient, these measurements may be used successfully in predicting problems such as abnormal formation pressure before a well control problem occurs. Lithology can also be inferred from certain types of measurements. However, the limitation remains that only surface information is available to infer conditions which may be far beneath the earth's surface.
Directional surveys of a borehole can be made presently by means of pump down or wireline tools. Here again, this is an after the fact measurement which requires significant interruption of the drilling process. Presently in commercial use are mud pulse systems for telemetering data from the bit vicinity to the surface, however, these systems are limited in their capability and application and as yet require the cessation of drilling.
There are basically four types of systems which show promise as communication and transmission techniques in a borehole telemetry system. These are mud pressure pulses, electromagnetic methods, insulated conductor or hardwire systems, and acoustic methods. Each of these systems has its advantages and disadvantages. The present application is concerned with an acoustical technique for transmitting signals through the drill pipe. This system offers a high degree of reliability together with a rapid data rate, and the potential of low development and production costs. The greatest obstacle to the development of such a system has been the very low intensity of the signal which can be generated downhole, along with the acoustic noise generated by the drilling system resulting in a high ratio of noise to signal. In order to overcome these problems work has proceeded in the development of a system utilizing repeaters in the drill pipe string to help alleviate the signal attenuation problem. As the development of this system proceeds, it is apparent that the use of discrete frequencies falling into particular band widths is essential for the successful transmission of acoustic data on a drill pipe. Accordingly, every means possible must be utilized in order to increase the efficiency of such a system to realize successful data transmission. One of the problems encountered in working with discrete frequencies is that of drift in the system which effects the synchronous use of precise frequencies. For example, temperature stability of components is a major problem together with the high cost of more stable components. This is aggravated by the severe temperature range which is encountered in drilling a well far beneath the earth's surface. In addition, the deterioration of components causes changes in circuit operations which in a precision system present problems. Another factor to be considered is that of the replacement of system cmponents and the affect that such replacement would have on the alignment of the system in view of the precision required in the systems.
It is therefore an object of the present invention to provide a new and improved acoustic telemetry system having stable circuit systems to permit the use of precise frequencies.